1. Field of the Invention
The present invention relates to magneto-optic memory media, and more particularly to a memory medium for use in memory devices, such as magneto-optic discs and magneto-optic cards, utilizing a magneto-optic effect.
2. Description of the Prior Art
Active research has been carried out on thin films of amorphous alloys comprising a rare-earth element and a transition metal for use as magneto-optic memory media. Among these, thin films of GdTbFe alloy composed of gadolinium and terbium as rare-earth elements and iron as a transition metal are in the range of 150 to 200.degree. C. in Curie temperature and have a suitable recording sensitivity for use as magneto-optic recording media with a semiconductor laser for recording, reproducing or erasing data.
However, the GdTbFe alloy usually has a small Kerr rotation angle of about 0.3 and therefore has the drawback of being unsatisfactory in the quality of signals reproduced. Accordingly, we have proposed a method of giving an increased Kerr rotation angle by using a dielectric film and a reflective film, i.e., by forming a dielectric film, GdTbFe film, dielectric film and reflective film in this order (see, for example, Applied Optics, Vol. 23, No. 22, p. 3927). With this four-layer film structure, the GdTbFe alloy film is made to have a reduced thickness of 10 nm to 50 nm to utilize both the Kerr effect and the Faraday effect. The Kerr rotation angle can then be increased easily threefold to fivefold to reproduce signals with an improved quality.
Nevertheless, the composition of the GdTbFe alloy is a further important factor in providing magneto-optic memory media of four-layer film structure which are satisfactorily usable in respect of the quality of reproduced signals, uniformity of size of recording bits and stabilized record retentivity.
The rare earth-transition metal alloy films for use as megneto-optic memory media are generally ferrimagnetic materials wherein the spins of the rare earth and the transition metal are in an antiparallel arrangement and which have a compensation point composition when the proportion of rare earth atoms is about 25 atomic %. FIG. 4 shows the relationship between the coercive force Hc of such films and the rare earth proportion thereof. The variation rate of the coercive force is smaller in compositions (RE-rich compositions) containing a larger amount of rare-earth atoms (RE) than the compensation point than in compositions (TM-rich compositions) containing a larger amount of transition metal atoms than the compensation point.
The coercive force, like the Curie temperature, is an important parameter influencing the recording sensitivity of magneto-optic memory media. The coercive force, when different at room temperature, also differs at the recording temperature as shown in FIG. 5, such that even if the medium is externally given the same recording magnetic field, data can be recorded at some portions but can not be recorded at other portions. In other words, if a memory medium varies in coercive force distribution along the plane thereof and is irradiated with a laser beam under the same condition for recording, the size (width) of bits recorded differs from portion to portion. Thus, the variation in the coercive force distribution is a primary cause of the degradation of reproduced signals. Variations in the coercive force distribution in the medium are mainly due to irregularities in the alloy composition of the medium and are therefore difficult to obviate.
Accordingly, in order to make the bit width uniform to the greatest possible extent and thereby preclude the degradation of reproduced signals, there arises a need to select a region of composition (e.g., the RE-rich composition region of FIG. 4) wherein the variation rate of coercive force is small relative to variations in composition.
The rare earth-transition metal alloy film for use as a magneto-optic medium must to have an axis of easy magnetization perpendicular to the plane of the film. When the coercive force is small, an intraplanar component of the axis occurs so as to make the recording bits unstable and impair the quality of reproduced signals. It is therefore desired that the GdTbFe alloy have a composition at least about 1000 oersteds in coercive force.
It is also known that with rare earth-transition metal alloy films, the Kerr effect and the Faraday effect at room temperature increase as the Curie temperature rises, giving reproduced signals of higher quality.
The foregoing indicates that magneto-optic memory media, in order to be useful, must have such a composition as to give signals of satisfactory quality.
In this connection, Laid-Open Dutch Patent Specification No. 7,713,503, FIG. 2 discloses a magnetic material which comprises an Fe-rich GdTbFe alloy represented by the formula (Gd.sub.x Tb.sub.1-x).sub.y Fe.sub.1-y wherein x is 0.65, and y is 0.23.
On the other hand, Examined Japanese Patent Publication SHO 60-32331 discloses a magneto-optic recording medium which comprises an Fe-rich GdTbFe alloy thin film represented by the above formula wherein x is 0.37, 0.50 or 0.81, and y is 0.27, 0.26 or 0.21.
The GdTbFe alloy films specifically disclosed in these publications afford reproduced signals which are satisfactory to some extent in quality when incorporated into magneto-optic memory media of the foregoing four-layer film structure. However, a further improvement in the quality of reproduced signals is desirable in avoiding reading errors and assuring reading at a higher speed.